The present invention relates to the fields of recovery of acids, in particular carboxylic acids, by means of amine extractants and of conversion of the recovered acids to derivatives. More particularly, the present invention relates to a process for the conversion of an extracted carboxylic acid to a non-ionized derivative thereof.
In the most general case these fields of recovery of acids by means of amine extractants and of conversion of the recovered acids to non-ionized derivativesxe2x80x94which fields, and techniques thereof, are known per sexe2x80x94provide for a sequence of two independent processes: the first for the recovery of an acid (as a crystalline product or an aqueous solution), and the second for the conversion of such recovered acid into a desired derivative such as salt, ester, polyacid etc.
These two fields, however, may be combined with benefit in cases in which the acid can be converted while still carried by a water-insoluble amine solvent, hereinafter referred to as the xe2x80x9cextractxe2x80x9dxe2x80x94and the acid conversion product recovered directly. Thus an extract of an acid that forms water-soluble salts can be reacted with an aqueous alkali in a single contact to form an aqueous salt solution and concurrently releasing the amine solvent (hereinafter referred to as the xe2x80x9cextractantxe2x80x9d). By such procedure the xe2x80x9cback-extractionxe2x80x9d operation of the acid recovery process, in which the acid is back-extracted by water, is eliminated.
U.S. Pat. No. 4,275,234 to Baniel et al. is illustrative of recovering carboxylic acids, as such, by means of extraction and back-extraction with an amine solvent. Similarily, U.S. Pat. No. 5,231,225 to Baniel et al. illustrates recovery by alkali. These examples are not unique. They fall within an extensive published prior art.
In analogy of converting acids to salts, directly in the extract, one could seek direct conversion to other derivatives. Indeed established chemistry provides for converting amine-bound acids to typical derivatives such as esters. This is particularly true of carboxylic acids which bond weakly to amines, such as trialkylamines of 20 C-atoms or higher typically used in Solvent Extraction. Thus citric acid can be converted to the ethyl ester, lactic acid can be converted variously to the ethyl ester, to polylactic acid and to lactide. A weak amine, similarly to hydrocarbons and other neutral solvents, is essentially inert with respect to such reactions. The term xe2x80x9cneutralxe2x80x9d as used herein is intended to denote that the solvent does not take part in the reaction itself though it may be assisting it directly or indirectly.
In order to illustrate the applicability of reactions in the presence of solvents that are neutral, the case of conversion of lactic acid to a variety of its non-ionised derivatives is considered.
U.S. Pat. Nos. 5,420,304 and 5,750,732, both to Verser et al. both teach processes for making cyclic esters of hydroxy-acids inclusive of lactide and polylactic acids from lactic acid in the presence of a great variety of neutral solvents. Such conversions have been known for many years to take place in the absence of solvents e.g. Carothers et al. and others quoted in the same patent. The solvents are thus obviously neutral in the sense given above to this word. The reasons for their use may be connected to reaction facilitation or to their presence due to having been used in the recovery of the carboxylic acid by Solvent Extraction. This last case, claimed by Verser et al., is of particular relevance to the present invention as explained in detail hereinafter.
U.S. Pat. No. 5,319,107 to Benecke et al. specifically describes xe2x80x9calkyl or aryl amine saltsxe2x80x9d (ibid page 5, lines 5-8) as preferred starting materials for making cyclic esters from hydroxy acids such as lactide from lactic acid. It should be noted that the term xe2x80x9camine saltsxe2x80x9d is an accepted way to describe the state of a carboxylic acid carried in an amine solvent phase such that is typically obtained by Solvent Extraction applying such an amine solvent.
EP 0 789 080 A2 to Kamm et al. titled: xe2x80x9cMethod for the preparation of organic ammonium lactates and their application in the production of dilactidexe2x80x9d claims a particular class of amines for purposes broadly defined in the previous cited patent.
The foregoing brings out the desirability of converting carboxylic acids, in particular carboxylic acids obtained in a solvent phase extract in a solvent recovery process, to product derivatives, while still carried in the organic phase of the extract. In addition to possible savings in back-extraction and subsequent operations, the very fact that the extract is free of water or nearly so can be of considerable advantage in reactions that involve water elimination such as esterification, lactonisation etc. as brought out in the cited patents and the numerous references cited therein.
The extensive use of these attractive possibilities is constrained however by the divergence between the demands of acid recovery by extraction with the demands of converting the acid as obtained in the extract. Typically the extract will contain acid at a low concentration level whereas for conversion of the acid a high concentration is desirable.
The last point relates to the sharp difference that obtains in converting carboxylic acids carried in an amine solvent extract to inorganic water soluble salts and their conversion to non-ionised organic derivatives such as esters. In the former case the concentration of the salt is determined by the concentration of the alkali used to react with the acid; in the latter case the solvent acts as a diluent which tends generally to affect negatively equilibria, reaction rates and separation costs.
The purpose of the present invention is to provide for simple ways to remedy this divergence, as well as to provide means that are easily adaptable to currently used industrial procedures.
With this state of the art in mind, there is now provided according to the present invention a process for the conversion of an extracted carboxylic acid to a non-ionized derivative thereof, comprising contacting a carboxylic acid containing aqueous solution with a water-insoluble amine solvent, to recover acid therefrom and to form an extract carrying amine-bound carboxylic acid, characterized in that said extract is contacted with a concentrated aqueous solution of said same acid of high concentration, whereby additional acid is transferred to said extract to form a loaded extract and said loaded extract is reacted to form a non-ionized derivative of said acid, by reactions known per se, whereby said acid is converted into a non-ionised derivative form and said amine solvent is liberated for recycling.
In preferred embodiments of the present invention said amine-bound acid extract is split into two streams, a first stream of which is back-extracted with water and concentrated to form said concentrated solution for recombination with said second extract stream.
In the present invention said loaded extract, carrying amine-bound carboxylic acid, may be reacted to form a non-ionized derivative of said acid by intra-molecular or inter-molecular reactions of the acid or reactions of the acid with a reagent, all of which reactions are known per se and do not form a part of the present invention.
While the invention will now be described in connection with certain preferred embodiments in the following examples and with reference to the accompanying figures so that aspects thereof may be more fully understood and appreciated, it is not intended to limit the invention to these particular embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the scope of the invention as defined by the appended claims. Thus, the following examples which include preferred embodiments will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purposes of illustrative discussion of preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of formulation procedures as well as of the principles and conceptual aspects of the invention.